Flatfish Welfare in Aquaculture: Turbot, Halibut and Sole
Flatfish Species in Aquaculture
Several flatfish species are farmed commercially: Atlantic halibut (Hippoglossus hippoglossus), turbot (Scophthalmus maximus), Dover sole (Solea solea), and Senegalese sole (Solea senegalensis). Halibut and turbot production is growing in Norway, UK, Spain, and Portugal. Sole farming is smaller-scale. Each species has distinct welfare needs reflecting their ecology. All are demersal (bottom-dwelling) fish with distinctive compressed body shapes.
Welfare Challenges
Key welfare challenges include: stress from handling (flatfish are very sensitive to air exposure and crowding); starvation during metamorphosis in hatchery production; skeletal deformities during early development; aggressive cannibalism in juveniles if size-grading is not maintained; eye migration failure in some individuals (causing cyclopia-like presentations); and sensitivity to light levels and substrate type. Flatfish require different husbandry to pelagic species.
Water Quality and Environment
Flatfish are bottom-dwellers and require adequate floor space in tanks or substrate in ponds. Water quality is critical: well-oxygenated, appropriate temperature (halibut: 12-14°C; turbot: 16-20°C; sole: 18-24°C). High ammonia concentrations cause gill damage. RAS (recirculating aquaculture systems) allow precise water quality control and are increasingly used for flatfish. Natural substrate (sand, gravel) reduces stress in sole.
Hatchery Welfare
Hatchery welfare is particularly challenging: flatfish larvae undergo metamorphosis from pelagic to benthic forms. During this period, mortality can be high. Provision of adequate live prey (rotifers, Artemia), appropriate light and turbulence levels, and prevention of starvation are critical. Some farms enrich live prey with HUFAs (highly unsaturated fatty acids) to improve larval nutrition and development.
Slaughter Welfare
Electrical or percussive stunning before slaughter is welfare-preferred. Live chilling in ice water or CO2 asphyxiation are poor welfare methods but remain in use. For premium markets (particularly halibut and turbot sold live or as sashimi), ike jime techniques are increasingly adopted. Industry awareness of flatfish welfare at slaughter is growing but implementation is inconsistent.